1. Field of the Invention
This invention relates to an exhaust gas analyzer used for continuously measuring particulate matters (PM) included in exhaust gas of a diesel engine of, for example, a vehicle.
2. Background Art
Minute particulate matters (PM) that might cause an adverse effect on the environment or human health are contained in exhaust gas of an internal combustion engine such as a diesel engine, and soot and soluble organic fraction (SOF) or the like are mixed in the particulate matters.
In order to improve the internal combustion engine so as to reduce the particulate matters, it is necessary to measure the PM emissions accurately. According to a method for measuring exhaust gas that is publicly specified by the requirement of current laws and regulations, it is specified that the PM is collected by a filter and the collected PM is weighted on a microbalance.
However, since it is impossible for this method to collect the generating PM dynamically, there is a demand for a simple measurement instrument that is capable of a real-time continuous PM measurement in a step of research and development of an internal combustion engine. Furthermore, recently since the PM emission is considerably decreasing due to improvement of the internal combustion engine, a demand for measuring a minute amount of PM emissions has been increasing in addition to the continuous measurement.
Regarding SOF, a method has been developed that can continuously and accurately measure concentration of liquefied or solidified particulate hydrocarbon (SOF) in exhaust gas at a certain reference temperature (47° C.±5° C.) by the use of a detector high-sensitive to hydrocarbon such as a flame ionization detector (FID).
Regarding soot, the inventors have been developing a high-sensitive soot detector that can perform continuous measurement by making use of a diffusion charge detecting method.
This kind of a soot detector requires that the exhaust gas be diluted to concentration appropriate to the exhaust gas measurement because it is high sensitive, however, there are the following problems in diluting the exhaust gas.
First, it is required to grasp a dilution ratio in order to calculate the concentration of soot in the original exhaust gas based on the concentration of soot in the measured diluted exhaust gas.
However, in order to obtain the dilution ratio if the soot detector has an arrangement wherein a flow of the exhaust gas prior to dilution and a flow of the exhaust gas after dilution are measured respectively by the use of a venturi meter and the dilution ratio is calculated based on the flow ratios, the soot detector becomes large and costly.
Secondly, it is preferable that the dilution ratio by the diluter can be changed with ease because the concentration of soot in the exhaust gas is affected to change by a variety or a state of the internal combustion engine.
However, if the exhaust gas as being sample gas or the diluent gas is forcefully fed or sucked by the use of, for example, a rotary pump in order for dilution, a bad influence might be exerted upon soot measurement such that an unexpected portion is clogged with soot because the fluid path inside the pump is complicated. In addition, the rotary pump is not suitable for accurate soot measurement because of pulsation generating in the diluted exhaust gas.
On the contrary, an ejector that conducts liquid transfer by making use of an involute action of fluid blowing out due to bounded jet has a simple flow path and there is no pulsation generating. Then the ejector can be used as a diluter that is very preferable for this kind of measurement.
However, generally it is considered that the ejector can not change a dilution ratio arbitrarily (if a lot of diluent gas is flowed, pressure in a nozzle diffuser part decreases and the flow volume of the sample gas increases by just this much and consequently the dilution ratio does not change significantly), the ejector has a problem with this point. Conversely, the ejector is often used in a case that the dilution ratio is to be kept to some extent or in a case that the dilution ratio is not cared at all (for example, a pump).
As mentioned, the ejector has a merit that the flow path is simple and free from pulsation, however, it is considered that it has a demerit that the dilution ratio can not be adjusted easily when used as a diluter. As a result, it is difficult to use the ejector for not only soot measurement but also other use such as the dilution ratio or the mixing ratio is required to be adjusted.